1. Field of the Invention
The invention relates to underground storage tanks generally, and more particularly to a method and apparatus for calculating underground storage tank buoyancy and buoyancy safety factors for various installations.
2. Discussion of the Background
Underground storage tanks (USTs) are commonly used for the underground storage of a wide variety of liquids, including the underground storage of fuels at locations such as gas stations. USTs are installed in a wide variety of locations and under a wide variety of conditions. In some locations, the water table is high enough such that some or all of the UST is below the water table. In these situations, a buoyant upward force will act on the tank. If the buoyant force exceeds the downward forces acting on the tank, the tank will xe2x80x98floatxe2x80x99 up out of the ground. This situation is obviously undesirable. Accordingly, it is necessary to determine the buoyant and downward forces acting on the tank to prevent this situation. Furthermore, some local governments require an installation to have a minimum buoyancy safety factor. The buoyancy safety factor is defined as the ratio of downward forces to upward forces. Thus, if a local government requires a safety factor of 1.2, then the installation requires downward forces acting on the tank to be 1.2 times greater than the buoyancy forces.
One source of downward force that acts on an installed UST is the backfill directly over the UST. As the burial depth increases, more backfill is placed over the tank and therefore more downward force acts on the tank. However, in some locations, it is impossible, impractical or prohibitively expensive to install a tank at a depth sufficient to compensate for buoyancy forces acting on the tank. Several schemes for increasing the downward force acting on the tank without increasing the burial depth are known in the art. One method is to form a concrete slab over the tank. A second method is to form a concrete slab below the tank and anchor the tank to the slab using straps or the like. A third method is to bury deadmen along with the tank and anchor the tank to the deadmen. An installation plan may employ one or more of these methods.
Ensuring that an installation plan for a single UST or multiple USTs is adequate is naturally of concern to UST installers. However, most tank installers do not have the knowledge and expertise to calculate the upward and downward forces to ensure that the installation plan is adequate. Many installers look to UST manufacturers to provide this information.
The Petroleum Equipment Institute has published an example on the calculation of buoyancy and buoyancy safety factors. The relevant publication is PEI 100-97, Recommended Practices for Installation of Underground Liquid Storage Systems, the contents of which are hereby incorporated by reference herein. The assignee of the present invention, Xerxes Corporation, has automated some of these calculations in the form of spreadsheets in the past. However, the example and previous Xerxes applications do not address multiple tank installations, and do not account for such variables as double-walled tanks having annular monitoring spaces that may be filled with air, may be maintained with a vacuum, or may be filled with brine or other monitoring fluids.
What is needed is a general method and apparatus for calculating tank buoyancy and buoyancy safety factors that can easily verify that adequacy of a UST installation plan.
The invention meets the aforementioned problems to a great extent by providing a method for calculating UST buoyancy and buoyancy safety factors that can be implemented on a computer. In one embodiment of the invention, installation plan information is entered over a medium such as the Internet. The computer then calculates the buoyancy and buoyancy safety factor and returns this information to the requesting party over the same medium. This technique allows installers to verify the adequacy of installation plans quickly. Records of the information provided by the installer may be kept so the recipient of the information (typically the UST manufacturer) can reconcile differences between the actual installation and the installation plan in the event of UST flotation. In another embodiment of the invention, the information may be provided on a paper form supplied by the installer. In preferred embodiments, the calculations are tailored to installation guidelines (often provided by the UST manufacturer), which may specify such parameters as spacing between tanks in multi-tank installations, some or all deadman dimensions, slab dimensions, etc. Preferred embodiments of the invention also have the ability to generate a form letter that includes the installation plan information and the results of the buoyancy and/or buoyancy safety factor calculations.